Peculiarities of CT Volumetric Imaging Towards the Optimal Image-Guided Radiotherapy

Abstract

Ensuring selective irradiation of target tissues is one of the biggest challenges in radiotherapy. Methods and devices of Image-Guided Radiotherapy (IGRT) are elaborated with the aim of ensuring that the prescribed radiation dose is delivered accurately to the tumour while minimising exposure to surrounding healthy tissues. Technical solutions ensure a few-millimetre, or even sub-millimetre precision of the irradiation beam, while with currently used mechanical means of patient positioning, we can expect much bigger positioning deviations, reaching even centimetre range. Patient positioning deviation to a certain extent is related to changes in soft tissue density and volume, which change during the period of treatment. Therefore, the discovery of reliable reference structures in routinely performed daily Cone-Beam Computed Tomograms (CBCT) was one of the aims of this study. Having the reliable reference structures, we carried out the retrospective estimation of patient position deviation during the whole treatment cycle and evaluated possible dynamics of unwanted irradiation of tumour-surrounding critical organs. The study was conducted in patients with head and neck cancer treated in the Lithuanian University of Health Sciences Kaunas Clinics Affiliated Hospital of Oncology, Department of Radiotherapy. Patients’ positioning was evaluated using volumetric images obtained by the CBCT machine integrated into the Halcyon V3.1 linear accelerator (Varian Medical Systems, Palo Alto, CA, USA). Custom-made algorithms of hard tissue segmentation and actual patient position estimation were elaborated in MATLAB (MathWorks, USA) environment. The hard tissue structures in volumetric images, in particular the mandible and part of the skull, were segmented and adjusted using mathematical morphology algorithms. We found these structures as reliable reference landmarks for patient position estimation. We found the deviation of actual patient position ranging from 1 to 3,5 mm, which resulted in changes in irradiation ranging from 0,016 to 0,057 Gy/fraction in the planned target volume and in critical surrounding organs (e.g. larynx, parotid, etc.) as well. The values indicate that it can cause significant damage to the surrounding organs. In conclusion, we state that specially selected hard tissue structures can serve as reliable landmarks of patient position, while soft tissues eventually change. The development of more precise image-guided radiotherapy methods can significantly reduce the damage to tumour-surrounding organs.